Reflow Soldering in Inert Atmosphere

By Oleg Mazurov

CO2 Soldering

Reflow soldering in toaster oven or on a skillet is popular among hobbyists and small-scale manufacturers. The process is simple and the equipment necessary is inexpensive and readily available. The results are very good if several simple rules are followed. One of these rules is to use freshly made PCBs – the shelf life of hot air leveled tin PCB finish is 6 months. A PCB which was used close to the end or past its shelf life will exhibit poor solder wetting resulting in bad joints.

Poor wetting can be overcome by increasing soak time and/or rising reflow temperature to give solder flux more time to act on oxide layer formed on the PCB finish during storage and reflow. This approach worked well in good old times of leaded solder, however, modern lead free solder reflow temperature is already quite high and increasing it may not be possible. In this case, soldering in inert atmosphere could be better option. In this article I will explain the process that I have started using some time ago – with great success.

Conceptually, soldering in inert atmosphere is easy: substituting air (which contains oxygen) in the oven with inert gas eliminates the cause of oxidation. The best gas for this purpose is nitrogen; it is non-reactive at soldering temperatures and being the major component of air it is the cheapest of all industrial gases. Being air-like it is also easy to use – all that is needed is a constant low volume flow into the oven during soldering cycle.
Nitrogen is available in several forms, the most suitable for soldering applications is compressed in a cylinder. Additionally, a regulator is needed to drop down high cylinder pressure. The price for the set starts at about $150. The cylinder can be refilled in places like Airgas for about $10.

Another gas suitable for creating inert environment is carbon dioxide or CO2. Unlike nitrogen, this gas has household uses. It is widely used to fertilize planted aquariums, carbonize beer, grow plants and several other things. Since I already own CO2 cylinders and regulators my reflow process uses this gas to create inert atmosphere. For volumes I use the difference in nitrogen/CO2 refill prices is insignificant.

The setup is very simple and is shown on the title picture. It consists of a toaster oven placed in a large plastic storage box, 5lb CO2 cylinder/regulator/needle valve, a length of silicone tubing and a box of matches. Since CO2 is heavier than air it won’t stay in the oven if pumped directly into it. Therefore, a container around the oven is needed. Since CO2 is not that much heavier than air it will raise when heated therefore a lid covering the container is helpful. A tube runs from a cylinder to the bottom of the box. Matches are used to check the inert atmosphere. I light a match and put it under the lid – if CO2 is there it will extinguish the match. Needle valve on the cylinder allows me to fine tune the flow, the heaviest gas loss will be during reflow peak when the temperature is the highest.

I tested the system with some pretty stale boards with were lying around for a couple of years. The wetting was excellent even with standard soak time of 3 minutes. For soldering normal PCBs no changes in profile were necessary. The overall process doesn’t change too much either: I place a board in the oven and start it, then I open the cylinder and keep it open until the oven temperature has dropped to about 170C after reflow peak. I haven’t noticed any change in oven dynamics, however, opening oven lid after the peak increases cooling rate.

If you already have a CO2 cylinder, this setup is inexpensive, quick to make and gives excellent results. I’m hoping to solder many boards with it – after about a hundred cycles the cylinder, which was about half empty when I started experimenting, still have plenty of CO2 in it. When I replace it with a fresh one I’ll start counting cycles to get a better idea of how long it will last.